Lord Krishna has told Bhagavad Gita to Arjuna on the first day of Bhishma Parva at Kurukshetra (before starting the war). And Karna was not present in Bhishma Parva as Karna had taken an oath not to fight till Bhishma is alive and acting as the Chief General of the Kaurava army. But Karna was explicitly mentioned by Duryodhana in the below verse from the Bhagavad-Gita Upa-parva of Bhishma Parva (BG 1.8):
Bhavaan bheeshmashcha karnashcha kripashcha samitinjayah;
Ashwatthaamaa vikarnashcha saumadattis tathaiva cha.8.
“Thyself(Dhronacharya) and Bhishma, and Karna and Kripa, the victorious in war; Asvatthama, Vikarna and Jayadratha, the son of Somadatta.
Does this mean Karna was present in the battlefield during Bhishma Parva and while Bhishma was still the acting General but he wasn't just involved in the war? If not, then why did Duryodhana mention Karna's name?